1. Field of the Invention
This invention relates to a plastic lens.
2. Description of the Prior Art
Plastic lens as have been widely used because they are lightweight, can be mass produced, and are generally low in manufacturing cost compared to conventional glass lenses. Materials for the casting of plastic lenses now in general use are diethylene glycol bis-allyl carbonate (to be hereinafter called CR-39) and the so-called acryl syrup of methyl methacrylate as the main constituent, and for the injection molding, polymethyl methacrylate (to be hereinafter called PMMA) and polycarbonate. The plastic lens materials in practical use are fairly fewer in kind than the conventional glass lens materials of which these are 200 or more kinds in total for casting and injection molding materials. While glass lenses have a considerably wide variation in index of refraction, ranging from high to low and also in dispersion, the plastic lens materials few in kind and have an index refraction of about 1.49 to 1.50 (exclusive of polycarbonate) i.e. much narrower width than for glass lenses, which is largely problematical for designing plastic lenses. For example, the lens for glasses smaller in index of refraction then the glass lens leads to a larger center thickness or curvature, thereby giving the appearance to a user different from that for glasses made of glass. Besides a combination of lenses different in the index of refraction or dispersion are used in camera lenses or microscope lenses for the purpose of eliminating various aberrations, such as chromatic, coma and spherical aberrations. Such method, however, is difficult to use with the plastic lens because of the few kinds thereof. Under these circumstances, other than the plastic lens of material now in use, those of various indexes of refraction or dispersions, such as high index or conversely low index of refraction, have earnestly been desired.
The plastic lens of PMMA, among plastic lens materials now in use, has a light transmission factor of about 92% in the region of visible light of wavelength of 400 to 700 m.mu. and also is made of a resin for general use and of a relatively low manufacturing cost, thus enjoying wide use. However, the PMMA has the largest drawback in low anti-breakage ability so that an improvement thereof has been an important object.
On the other hand, from the viewpoint of the resin molding, in case of manufacturing the plastic lens by the injection molding, while it is advantageous in the fairly short time taken for the molding in comparison with the casting, there are drawbacks fundamental for the injection molding such that optical strain caused by molecular orientation of resin is generated in the lens, the lens is easily flawed due to the use of thermoplastic resin, the heat-resistance temperature is limited, and materials usable for injection molding are restricted.
On the contrary, in the case of manufacturing the plastic lens by the casting, the blending is selected relative to a degree of freedom to thereby permit manufacture of lens having a desired index of refraction. On the other hand, it takes much time to cure liquid materials, for example, the CR-39 requires several tens of hours for curing.
In order to eliminate the above defect in the casting, an attempt has been made to cure resin by irradiation with ultraviolet rays to manufacture the plastic lens. This method employs a photosetting type resin containing as the main constituents a transparent compound, e.g., methyl methacrylate, transparent with respect to the visible light and photopolymerization initiator transparent with respect to the visible light, the photosetting resin being poured into a transparent mold having a desired optical surface, and thereafter the ultraviolet rays are radiated to the resin through the transparent mold to thereby cure the photosetting resin. In this case, however, the cured resin generally changes in color from colorless to yellow following the curing. If the coloration becomes a large drawback to the performance of lens, the plastic lens having used the photosetting resin is not practical. Hence, a plastic lens has been keenly desired which employs a photosetting resin and which, when cured, creates no coloration during the curing by the ultraviolet ray radiation.